The invention relates to densifying porous substrates with pyrolytic carbon (PyC) obtained by chemical vapor infiltration (CVI).
Densifying porous substrates by PyC-CVI is well known. In a commonly used method, substrates for densifying are loaded into an oven, and a reaction gas containing at least one PyC precursor reagent gas together with a vector gas is admitted into the oven. An effluent gas containing residual components of the reaction gas and reaction by-products is removed from the outlet of the oven. Operating conditions, in particular the temperature and pressure inside the oven, the content of precursor reagent gas in the reaction gas, and the transit time of the reaction gas through the oven, are selected so as to form the desired PyC deposit within the pores of the substrates.
The precursor reagent gas comprises at least one hydrocarbon of the CxHy type, where x and y are natural integers and x≧2, e.g. propane C3H8 or butane C4H10, or indeed propylene C3H6. Its function is to be the main contributor to the PyC deposition reaction, and it is sometimes referred to as a dopant gas.
The vector gas has a dilution function encouraging the reaction gas to diffuse into the core of the pores of the substrates. It may be constituted by an inert gas such as nitrogen N2, helium He, argon Ar, or indeed methane CH4. Methane reacts little and acts more as a vector gas than as a PyC precursor gas.
It is also known from U.S. Pat. No. 5,789,026 to add hydrogen gas H2 into the reaction gas in order to modulate deposition kinetics, where H2 performs a slowing down function in a certain range of temperatures.
This function of H2 in a PyC-CVI type densification process is also described in U.S. Pat. No. 6,197,374 which recommends using a gas made up of methane CH4, and of hydrogen H2, using pressures that are high relative to the usual pressures, and not using any vector gas or dilution gas. Recycling a stream of gas extracted from the effluent gas is envisaged, but hydrocarbon compounds heavier than methane are eliminated. The method described appears to be of an experimental rather than of an industrial type.
The possibility of recovering components from an effluent gas in a PyC-CVI densification process is mentioned in U.S. Pat. No. 5,348,774, whether for recycling or to produce energy, but without any other details on how the recycling might be implemented.
Known PyC-CVI type methods presently in use are expensive in terms of their consumption of the gases constituting the reaction gas, and also in terms of energy consumption.
It is therefore desirable to have a CVI method that enables a significant reduction to be achieved in the costs of densifying porous substrates with PyC.